1. Field of the Invention
The present invention generally relates to a method useful for determining the stress at the borehole boundary at a selected location within a borehole. The present invention comprises a method for determining the stress by measuring a plurality of parameters indicative of the formation at the selected location, calipering the deformed borehole at that location and determining the stress at the borehole boundary at the selected location employing the information obtained by measuring and calipering.
2. Description of the Background
In situ formation stresses are very important to the geologist, petrophysicist, drilling engineer and production engineer. These stresses affect borehole stability both while drilling and during production. These stresses can lead to borehole closure, collapse, spalling or enlargement, resulting in significant drilling and production problems. For example, the size of the borehole directly impacts the performance of bottom hole assemblies used in directional drilling. High stress levels which cause closure of the borehole affect casing integrity and may lead to casing collapse. The competence of the formations in the completion interval is very important with regard to sand control and the productivity of the reservoir. These examples are merely illustrative of the many problems resulting from the stress at the boundary of the borehole.
There has been no effective way of determining borehole stress in the field. Mathematical models have been developed to determine the effects of hypothetical stress levels on borehole stability. For example, Gnirk has addressed the elastic/plastic behavoir of uncased well bores under hydrostatic stress. Gnirk, "The Mechanical Behavior of Uncased Wellbores Situated in Elastic/Plastic Media Under Hydrostatic Stress", Society of Petroleum Engineers Journal, Feb. 1972, pp. 49-59. Further, Bradley has addressed the theoretical aspects of borehole failure caused by hydraulic fracturing or borehole collapse. Bradley, "Failure of Inclined Boreholes", Transactions of the ASME, Vol. 101, Dec. 1979, pp. 232-239; Bradley, "Mathematical Concept -- Stress Cloud Can Predict Borehole Failure", The Oil and Gas Journal, Feb. 19, 1972, pp. 92-102; and Bradley, "Predicting Borehole Failure Near Salt Domes", The Oil and Gas Journal, Apr. 2, 1979, pp. 125-130.
Tools have long existed which are capable of calipering a borehole. For example, in U.S. Pat. No. 3,969,929, Shaw disclosed a device useful for determining the stress in the wall of a borehole by recording successively on the same holographic recording film two holograms of the selected surface region of the wall. During the interval between the recording of the two holograms, a stress relief hole was drilled in the wall of the selected surface region. Calipering methods and apparatus include mechanical calipers suitable for incorporation within a drill string such as those disclosed by Montgomery in U.S. Pat. Nos. 2,719,361, 2,719,362, and 2,719,363. Serata in U.S. Pat. No. 4,149,409 disclosed a borehole stress property measuring system including a wireline device for measuring the borehole geometry at a selected location before and during the application of a known stress applied by hydraulically actuated sleeves.
The devices and methods referenced above do not permit the immediate determination of stress affecting the borehole boundary based on parameters readily measurable during the drilling operation. Those devices and methods determine stress by repetitive measurements of the borehole geometry before and after a stress effecting event, e.g., stress relief or application of additional stress. Accordingly, the drilling engineer, geologist, petrophysicist and production engineer remain relatively unaware of the borehole stress and its practical affects on the stability of the borehole boundary during drilling.
Accordingly, there has been a long felt but unfulfilled need within the borehole drilling industry for an apparatus and method useful in determining the stress at the borehole boundary at a selected location within the borehole based on the measurement of a plurality of parameters indicative of the formation and on the geometry of the deformed borehole at that location.